Electrically heatable laminated glass plates with a continuous, transparent surface coating and bus bars in electrical contact therewith are known in numerous different constructions. It is known from U.S. Pat. No. 4,668,270 to first place on the glass surface bus bars composed of an electrically conductive heat curable paint, and then apply the transparent surface coating serving as the heating resistor to the glass surface and the bus bars. Power connection elements are electrically and conductively connected to current leads on the bus bars.
It is also known to initially apply the transparent electrically conductive surface coating to the glass surface and to apply the bus bars by printing an optionally heat curable conductive paste onto the surface coating, as disclosed in U.S. Pat. No. 4,830,876.
U.S. Pat. No. 4,820,902 discloses initially placing a decorative frame of an opaque heat curable paint on the inner surface of the outer glass plate (facing the thermoplastic intermediate layer), then printing the bus bars on the decorative frame from an electrically conductive heat curable paste. The surface coating serving as the heating resistor is then applied using a vacuum process. The current connecting cables are connected to the printed-on bus bars.
It has also been found desirable, when a construction such as that last mentioned is employed, to provide marginal cutouts on the second glass plate in the areas where the cables are connected to the bus bars on the first glass plate. This technique, shown in U.S. Pat. No. 4,654,067, thus provides space in the second plate into which the connections on the first plate fit.
It is also known in the art to initially apply the surface coating serving as the heating resistor to the glass surface and to use metal foil bands or strips for contacting purposes. These bands or strips are connected by means of an electrically conductive adhesive layer to the surface coating, as taught by U S. Pat. No. 3,612,745.
All of these prior art electrically heatable laminated glass plates with transparent heating resistor coatings, and all known processes for producing them, suffer from specific disadvantages. For example, in the case of contacting the heating resistor coating by metal foil strips, damage or even complete destruction of the heating resistor coating frequently occurs immediately alongside said strip during heating, because dangerous voltage and current peaks occur in the immediate vicinity of the strip. When contacting the heating resistor coating with the aid of printed-on, heat cured bus bars, the problem arises that either metallic plug contacts or cable portions must be soldered to the bus bars, but the soldered joints are located directly on the glass face and corresponding marginal cutouts must therefore be made in the second glass plate. This not only involves additional labor costs, but also leads to significant problems in connection with the sealing and fitting of the glass plate in the car body window frame.
It is therefore an object of the present invention to provide an electrically heatable car glass plate or windshield of the aforementioned type, in which the heating current is uniformly led to the resistance coating through bus bars without any risk of damage or destruction to the coating as a result of local voltage and current peaks.
It is a further object of the invention to provide such a glass plate or windshield in which connection of the bus bars within the laminated glass plate to the current leads outside the glass plate requires minimum effort and expenditure, without impairing the seal of the laminated glass plate on the edge and without making it more difficult to install the glass plate.